1. Field of the Invention
The present invention relates to a plasma processing apparatus using a surface wave plasma.
2. Description of the Related Art
Plasma technique has been heretofore often used in semiconductor manufacturing processes for coating, etching, ashing, and the like. In addition, the plasma technique has been also used for manufacturing solar cells, liquid crystal displays, plasma displays, and the like. Recently, in accordance with the increase in size of a panel or a screen, a plasma processing technique of a large area has become important. For this purpose, it is necessary to generate a plasma which is large and has a uniform plasma density. If the plasma density is uniform, uniform plasma processing can be applied to an entire surface of a substrate.
As a plasma processing apparatus which can generate a large plasma, there is an apparatus using a surface wave plasma (SWP) (e.g., see JP-A-2000-348898 (page 2, FIG. 1)). In this apparatus, microwaves being transmitted the inside of a waveguide are introduced into a plasma generating chamber from a slot antenna through a dielectric window (microwave transmitting window) so that a process gas in the plasma generating chamber is excited by surface waves generated at a surface of the dielectric window facing the plasma generating chamber to thereby generate a plasma. Since the surface waves are transmitted on the plasma generating in chamber of the dielectric window promptly, the plasma tends to expand, and a large plasma area can be obtained easily.
In this type of a plasma processing apparatus, plasma processing is performed while a pressure in a vacuum chamber (that ism the plasma generating chamber) is kept at about 0.1 to 50 Pa. In order to maintain this decompressed atmosphere, a vacuum seal is applied in a place close to an external circumference of the dielectric window.
In the case in which the vacuum seal is applied in the place close to the external circumference of the dielectric window, a maximum stress is loaded in a central part of the dielectric window. As described above, an area of the dielectric window has to be increased in order to obtain a plasma of a large area. When the area of the dielectric window is increased, the maximum stress increases more. Therefore, it is necessary to replace a material of the dielectric window with a material having a high strength or to increase a thickness of the dielectric window.
However, a high-strength dielectric material such as aluminum or zirconia has a disadvantage that it is expensive and hard to be processed. On the other hand, it is not desirable to increase the thickness of the dielectric window because its own weight and material costs increase or it becomes susceptible to thermal shock due to a plasma.